These days, I agree the likeliest source of a locked homepage is one of your security programs, but this is not limited to the AVs pushed by Dell, by any means.

 

So if you can't find the way to unlock your home page from within your AV/security suite,  probably the easiest thing to try is to reboot into " Safe Mode with Networking", and you should be able to change your home page there via the usual means (IE7>Tools>Internet Options etc)

 

And if that doesn't work, a more complete set of causes and fixes are outlined here:
http://naut.homestead.com/files/locked/locked.html

 

 

So, changing your homepage on the Internet Options General window doesn't work? I have IE 7 installed on my system in both operating systems without any problems.

 

DIM 4700, Intel P4 3.0 Ghz, 2.5GB RAM, Dual Boot Win XP MCE 2005 Service Pack 2, Windows Vista Ultimate x32 bit
BIOS version A10
Maxtor Diamond Plus9 6Y080M0 80GB internal SATA HD
Seagate Barracuda 7200.9 ST3250824AS 250GB internal SATA HD
TSST corp TS-H352C DVD-ROM
TSST corp TS-H552B DVD+RW
Intel PRO/100 VE Network Adapter
ZyDas 802.11b/g USB 2.0 Wireless adapter
HP Photosmart 7850 Printer
Dell E173FP Monitor

 

Check the configuration of your security suite (Mcafee, Norotn) becasue there is a setting in there that is not allowing you to change this.  It is seeing your change as spyware.